Optical disks are often inked for identification, advertising or other purposes. During the inking process, an optical disk, such as a compact disk (CD) or digital video disk (DVD) are passed through a printing machine. The disk is attached to a carrier to protect and secure the disk. Then, the carrier and disk are put through one or more inking steps. Each step can include a different color ink. Most printing machines include a screen tear-off sensor that is a device that detects ink, usually, white ink, on the disk carrier of the printing machine.
A tear in a screen used for printing (i.e., a screen tear-off) or other inking errors may cause ink to fall on a print table, the carrier or collect too much ink in one area of the disk. That is, a screen tear-off, for example, will cause the print table to be stained with ink. In conventional machines, such as offset printing machines, a sensor is installed which is intended to detect white base screen tear-off printing errors. The operation of this sensor is based in the change of height that would occur in the machine print table caused by the white ink falling off of a printing screen at the time of a screen tear-off. More specifically, such a sensor is blocked by ink on the print table, looking at it side-to-side.
Referring to FIG. 1, a high level block diagram of a cross-section of a relevant portion of a screen offset printing machine 10 is illustratively depicted. Machine 10 includes a housing 12. Housing 12 incorporates a screen tear-off sensor 14, which receives light 18, preferably laser light or LED light, from a source 16. During printing, white ink 22 is applied to a surface of an optical disk 24.
Currently, offset printing machines come with a sensor similar to sensor 14, which is intended to detect white base screen tear-offs. The purpose of this sensor 14 is to detect a change in the height that would occur on a machine print table 20 caused by white ink falling off a screen 11 in housing 12 at the time of a screen tear-off (e.g., the sensor 14 is blocked by ink on the print table 20, from a side position). However, not all screen tear-off ink errors cause a change of height large enough and long enough for the sensor to detect, thus causing the screen tear-off to be unnoticed by the machine control.
Such unnoticed ink errors can cause offset head rollers used in the printing process to become dirty with ink. The offset head rollers need to be cleaned immediately to avoid further print defects that would affect product quality caused by the staining of subsequent disks with ink from the offset head rollers. Disks with such ink defects due to the failure to detect the ink errors in printing immediately, need to be scrapped. Even further, and each occurrence typically causes more than an hour of machine downtime. Although printing machines automatically stop when the disks reach the end of the printing machine due to an included print scanner inspection in the printing process, by the time a first disk having an ink error is detected by the print scanner inspection, many subsequent discs already have ink defects.
Furthermore, in such prior art printing systems including ink error sensors, such as the screen offset printing machine of FIG. 1, the sensors 14 require accurate calibration, which can be a sensitive and lengthy process, and even if properly aligned, many screen tear-off ink errors will not be noticed by the sensor.
Therefore, a need exists for a system and method, which provides improved detection of screen tear-off conditions and other ink errors on at least a printing table of disk printing systems.